Cathodes for fluorine gas discharge lasers

ABSTRACT

A fluorine gas discharge laser electrode for a gas discharge laser having a laser gas containing fluorine is disclosed which may comprise a copper and copper alloy cathode body having an upper curved region containing the discharge footprint for the cathode comprising copper and a lower portion comprising a copper alloy, with the facing portion of the electrode if formed in a arcuate shape extending into straight line portions on either side of the arcuate portion, the straight line portions terminating in vertical straight sides, with the boundary between the copper including at least the arcuate portion, the electrode may comprise a bonded element machined from two pieces of material the first made of copper and the second made of a copper alloy bonded together before machining. The electrode may also comprise a first and a second elongated lopsided V-shaped groove formed along substantially all of the elongated electrode body forming a discharge receiving ridge between the first and second lopsided V-shaped grooves, with a differentially faster eroding material filling the first and second lopsided V-shaped grooves. also disclosed is an electrode system in which the one electrode, e.g., the cathode bows during operation and may comprise at least one of a first and second elongated gas discharge electrode being machined to form a crown to receive the gas discharge that compensates for the bowing of at least one of the gas discharge electrodes during operation of the fluorine gas discharge laser.

RELATED CASES

This application is a divisional of U.S. patent application Ser. No.10/672,181, filed Sep. 26, 2003, which is a continuation in part of U.S.patent application Ser. No. 09/953,026, filed on Sep. 13, 2001, entitledDISCHARGE LASER WITH POROUS INSULATING LAYER COVERING ANODE DISCHARGESURFACE, with inventors Morton, et al., published on May 2, 2002, Pub.No. US20020051478 A1; U.S. patent application Ser. No. 10/081,589,entitled ELECTRIC DISCHARGE LASER WITH TWO-MATERIAL ELECTRODES, filed onFeb. 21, 2002, with inventors Morton et al., published on Oct. 24, 2002,Pub. No. US2002015467AI0; U.S. patent application Ser. No. 10/104,502,entitled HIGH REP-RATE LASER WITH IMPROVED ELECTRODES, filed on Mar. 22,2002, with inventors Morton, et al., published on Dec. 19, 2002, withPub. No. US20020191661A1; U.S. patent application Ser. No. 10/629,364,entitled HIGH REP-RATE LASER WITH IMPROVED ELECTRODES, filed Jul. 29,2003; U.S. patent application Ser. No. 10/638,247, entitled HIGHREP-RATE LASER WITH IMPROVED ELECTRODES, filed Aug. 7, 2003; thedisclosures of all of the above being hereby incorporated by reference.

This case is also related to Attorney Docket Nos. 2003-0048, entitled“ANODES FOR FLUORINE GAS DISCHARGE LASERS,” and attorney Docket No.2003-0058, entitled “ELECTRODE SYSTEMS FOR FLUORINE GAS DISCHARGELASERS,” filed on the same day as this application and assigned to thecommon assignee of this application, the disclosures of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The field of the invention relates to electrodes and electrode systemsfor fluorine gas discharge lasers.

BACKGROUND OF THE INVENTION

The above referenced previously filed co-pending applications relate tovarious aspects of electrodes, particularly for electrode systemsutilized in gas discharge lasers, and more particularly gas dischargelasers utilizing a laser gas containing fluorine, referred to asfluorine gas discharge lasers. In addition U.S. patent application Ser.No. 10/243,102, fled on Sep. 13, 2002, entitled TWO CHAMBER F2 LASERSYSTEM WITH F2 PRESSURE BASED LINE SELECTION, with inventors Rylov, etal., published on Jul. 24, 2003, with Pub. No. US20030138019A1, U.S.patent application Ser. No. 10/210,761, filed on Jul. 31, 2002, entitledCONTROL SYSTEM FOR A TWO CHAMBER GAS DISCHARGE LASER, with inventorsFallon et al., published on Feb. 13, 2003, with Pub. No.US20030031216A1; U.S. patent application Ser. No. 10/187,336, filed onJun. 28, 2002, entitled SIX TO TEN KHZ, OR GREATER GAS DISCHARGE LASERSYSTEM, with inventors Watson, et al., published on Jan. 16, 2003, Pub.No. US20030012234A1, and U.S. Pat. No. 6,584,132, entitled SPINODALCOPPER ALLOY ELECTRODES, issued to Morton, on Jun. 24, 2003 discussvarious aspects of fluorine gas discharge lasers and electroderequirements for such lasers as well as other laser life, particularlychamber life issues surrounding the operation of such lasers.

It is well known, as the above references discuss that the environmentfor electrodes in a fluorine gas discharge laser is complex and severe.Increasing requirements for output laser power, resulting in, amongother things, higher voltages across the electrodes, and higher totalpower dissipated in the discharges over electrode life, exacerbating theseverity of he gas discharge laser chamber environment. The need toincrease pulse repetition frequencies well above 4000 Hz, and even up todouble that repetition rate during pulse bursts, equally causes problemsin maintaining electrode lifetimes. The need for more pulses per burstand other well known and increasing severe demands on the gas dischargelaser electrodes, particularly in fluorine gas discharge lasers has leadto and will continue to lead to demands for improvements in electrodeand electrode assembly technologies. Some of which are more specificallydirected to cathodes, and/or their assembly as part of the laser chamberand some more specifically to anodes and/or their particular assembly.The electrical, electromagnetic, physical and chemical influences onelectrode lifetimes continually place challenges on the designs forelectrodes and their interfaces with other parts of the chamber,including the gas discharge region between the electrodes themselves.The present application addresses some of the above noted concerns.

SUMMARY OF THE INVENTION

A fluorine gas discharge laser electrode for a gas discharge laserhaving a laser gas containing fluorine is disclosed which may comprise acopper and copper alloy cathode body having an upper curved regioncontaining the discharge footprint for the cathode comprising copper anda lower portion comprising a copper alloy, with the facing portion ofthe electrode if formed in a arcuate shape extending into straight lineportions on either side of the arcuate portion, the straight lineportions terminating in vertical straight sides, with the boundarybetween the copper including at least the arcuate portion, the electrodemay comprise a bonded element machined from two pieces of material thefirst made of copper and the second made of a copper alloy bondedtogether before machining. The electrode may also comprise a first and asecond elongated lopsided V-shaped groove formed along substantially allof the elongated electrode body forming a discharge receiving ridgebetween the first and second lopsided V-shaped grooves, with adifferentially faster eroding material filling the first and secondlopsided V-shaped grooves. also disclosed is an electrode system inwhich the one electrode, e.g., the cathode bows during operation and maycomprise at least one of a first and second elongated gas dischargeelectrode being machined to form a crown to receive the gas dischargethat compensates for the bowing of at least one of the gas dischargeelectrodes during operation of the fluorine gas discharge laser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a graph of anode profile changes over Bp;

FIG. 2 shows a graph of cathode profile changes;

FIG. 3 shows a typical axial anode erosion profile;

FIG. 4 shows a graph of cathode and anode erosion rates;

FIG. 5 shows cathode discharge width change for different materials;

FIG. 6 shows a graph of surface roughness vs. alloy type;

FIG. 7 is a graph illustrating cathode erosion;

FIG. 8 is a graph showing worn anode and cathode surface morphology v.the composition of the material for an anode;

FIG. 9 shows an illustration of cathode related surface chemistrychanges following exposure to a laser chamber gas discharge;

FIG. 10 shows an electrode system exemplifying differential erosion;

FIG. 11 shows a bonded bi-metallic cathode;

FIG. 12 illustrates a diffusion bond;

FIG. 13 shows a prior art example of an anode;

FIGS. 14 a-f illustrate an exemplary artificial reefing process;

FIG. 15 shows a possible electrode configuration;

FIGS. 16 a-c show multi-metallic electrodes having improved and tunablethermal conductivity properties;

FIG. 17 shows a cross-sectional view of an electrode arrangement;

FIG. 18 shows a multi-segmented electrode;

FIG. 19 illustrates another cathode;

FIGS. 20 a-c illustrate an anode and cathode system with improved longlife discharge shape control capabilities;

FIGS. 21 a-b illustrate schematically a reef templating process;

FIG. 22 shows the proportions of ally materials in brass that allow orinhibit reefing;

FIGS. 23 a-d illustrate a bowed electrode concept;

FIG. 24 illustrates a structure for reducing erosion at the ends of anelectrode;

FIG. 25 illustrates a tilted crown discharge region electrode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Cathode erosion in fluorine containing gas discharge chambers for gasdischarge lasers comprising a laser gas containing fluorine, e.g.,Krypton Fluorine (“KrF”) or Argon Fluorine (“ArF”) and molecularfluorine (“F2”) lasers (herein referred to a “fluorine gas dischargelasers”) is very different from anode erosion. Anode wear is dominatedby F⁻ ion erosion corrosion, while cathode wear is dominated by highcurrent ionized noble gas ion ablation. In ArF excimer laser chambers,e.g., applicants observed that cathodes erode much more slowly thananodes. Cathode erosion rates have been found by applicants to betypically less than 30% of anode erosion rates. For brass alloys,cathode erosion rates have been found to trend relatively linearly withthe Zn content in the alloy.

Applicants believe this linear trending with the Zn content of the alloyis due to a combination of the mechanisms of Zn vaporization andpreferential sputtering from the parent alloy. In contrast to anodecorrosion, cathodes do not naturally grow self-passivating metalfluoride layers. Therefore, it is not possible to rely on a naturallygrown “reef”-like coating to passivate the alloy surface and protect acathode from erosion. Due to energy and discharge stabilityrequirements, slow eroding alloys appear to outperform engineeredpassivated/coated systems, i.e., attempts to artificially grow ordeposit “reef”-like self passivating material on the cathode. Inaddition, cathode wear morphology (pitting and roughness changes duringuse) appears to impact the erosion rates of the opposing anode.Applicants have determined that there is a clear interaction of cathodeand anode regarding material erosion rates. Therefore, it is desirablefor both the anode and cathode to be a compatible pair for optimumfluorine gas discharge laser chamber lifetimes.

In general, single phase materials such as Ni, and low meltingtemperature brass alloys (e.g., Zn levels >20%) appear to be better foranode erosion rates. Cu—Zn cathode alloys containing less than 15% Znappear to wear more slowly than high Zn containing brass alloys, howeverthey appear to pit as cathodes, apparently then accelerating anode wear.In general, the ideal cathode material will both erode slowly and stayrelatively smooth over the chamber lifetime.

As part of applicants' efforts to prolong chamber lifetime, and morespecifically electrode life, in applicants' assignee's products, e.g.,in 7XXX laser products, applicants have evaluated candidate alloys forboth long life anodes and cathodes. In addition, applicants haveinvestigated optimizations for composite cathodes (e.g., fordifferential erosion) manufactured, e.g., via bonding processes, e.g.,diffusion bonding of Cu alloys. A series of 2 KHz laser pulse repetitionrate segmented anode and cathode tests were run. In these tests,applicants investigated both anode and cathode corrosion rates, e.g., asa function of alloy composition. Older style fluorine gas dischargechambers were utilized, e.g., 6XX0 chambers as have been utilized inapplicants' assignee's 6XX0 products were built and run using segmentedanodes or cathodes. By using multi-component anodes and cathodes,applicants were able to make “apples to apples” comparisons of materialerosion rates and to investigate anode/cathode interactions.

Using baseline 2 KHz KrF and ArF laser chambers for purposes ofconducting the evaluations. In order to minimize electrode geometryeffects, the same anode and cathode shapes were used for all tests. AllArF chamber tests used the same cathode part as are used in applicant'sassignee's ArF products and an anode, also utilized in applicants'assignee's ArF products. The KrF chambers used a KrF cathode and asegmented anode shapes similar to those used in the ArF tests. For thesegmented cathode tests, a ArF brass (C26000) anode was used opposite acathode with 7 sample alloys fastened onto the C26000 substrate. Allchambers were tested using standard gas fills for applicants' assignee's6010A and 6010K products for a minimum of 2 billion pulses (“Bp”) at 2to 2.5 KHz gas discharge repetition rates on applicants' assignee'slaser frames. Segmented electrode tests were run at fixed voltage(,e.g., 1100 volts), and operated in an optics-free mode. After thetests were completed, electrodes were removed, photographed, and erosionwas measured via a NIST calibrated dial caliper (0.01 mm resolution).

In order to quickly investigate the effects of Zn, Cu, and Pb onelectrode wear, a variety of readily available commercial copper alloyswere selected by applicants for testing. Since alloy manufacturing wasnot under process control, all Cu alloy samples were heat treated andsent out for external chemical and metallurgical characterization. Thiswas to investigate what impurities, if any, were typically in these Cualloy systems. In general, Cu alloys are frequently recycled, thereforemetallic and dissolved gas impurity levels are not well controlled. AllPb containing test alloy samples were annealed at 900 F to avoidhot-shortness (melting of the tertiary Pb alloy phase, which can causevoid formation in the material), and non-Pb containing alloys wereannealed at 1200 F. Special Zn and Ni alloys were not annealed. Table 1.Below, summarizes the alloys investigated in the segmented electrodestudies. Total metallic impurities were measured via glow discharge massspectrometry (“GDMS”) by wt. High impurity values suggest extensiverecycling of an alloy. TABLE 1 Summary of Metal Alloys Tested in 2 KHzSegmented Anode and Cathode Chambers (TD094, TD095, TD98, TD103, TD106)ArF Tests KrF Tests Composition Grain Impurities Alloy # (A or C) (A orC) (Wt %) Size (PPM wt)  1 A and C A and C Cu-99 125 350  2 A and C Aand C Cu-99, Zn-5 90 225  3 A and C A Cu-90, Zn-10 89 N/A  4 A and C Aand C Cu-70, Zn-30 50 350  5 A and C A Cu-60, Zn-40 45 554  6 A and C ACu-89, Zn-8.7, 40 >3500 Pb-1.6  7 A A Cu-61.5, Zn-37, >200 >1000 Pb-1.5 8 A and C A and C Cu-61.5, Zn-35.5, 15 >10000 Pb-3  9 A A Cu-61.7,Zn-38, >200 >5800 Pb-.3 10 A A Cu-59, Zn-38, >200 >700 Pb-3 11 A ACu-76, Zn-15, >200 >500 Pb 6, Sn 3 12 A A Pb-99.9 N/A N/A 13 A and C ANi-99.9 >50 N/A 14 (GM) A A Zn-91, Cu-5.5, N/A N/A Al-3.0 15 A A Zn-91,Cu-1, N/A N/A Al-8

The above candidate alloys were selected to investigate the roles of Cu,Zn, and Pb on alloy erosion in both anode and cathode applications.Historical data had suggested that, e.g., free machining brass andcartridge brass alloys have been extensively used as chamber anodes andcathodes, therefore Cu, Zn, and Pb were chosen as critical alloyingelements for study by applicants.

The metallurgy of brass is not very complicated. As Zn is added tocopper, the material forms a 2 phase (banded) microstructureincorporating the Zn and Cu within the crystal grains. If Pb is added, a3 phase microstructure develops since it is not soluble in Cu and Zn. Pbinclusions develop in Pb containing brasses during annealing at grainboundary interfaces. The grain boundaries, e.g., for free machiningbrass, have roughly 20 micron length and width dimensions and the leadinclusions form roughly 5 micron pockets at the junctions of severalgrain boundaries.

In general, Zn can be safely increased to 42 Wt % in brass alloyswithout creating materials that are brittle and/or difficult tofabricate into components. Zn helps strengthen the Cu alloy by reducingslip planes in the Cu microstructure. It also increases the sputteryield and vapor pressure of the Cu alloy, while also reducing meltingtemperature. Since Zn is also more reactive with Fluorine than Cu, therole of zinc in electrode erosion is of great interest.

Applicants also investigated the effects of cathode wear morphology onanode erosion, i.e., anode-cathode wear interactions. Applicants havebeen aware that cathode segments opposite self-passivating alloys(“reefing alloys) appear to wear quickly, and that anodes wear fasteracross from cathodes that pit. Applicants have given theseself-passivating structures the name “reef” because of the generalappearance of a coral reef formed on the substrate electrode material inthe self-passivating process and the terms are used interchangeably inthis application.

Segmented electrode tests are a way that anode cathode interactions canbe investigated in realistic time frames. In order to quantify electrodesurface changes, cathode samples were evaluated for roughness changes bya so-called “Pocket Surf” Federal Profilometer, e.g., made byMahr-Federal Inc. of Providence, R.I., and chemically investigated bySEM-EDX analysis.

Applicants have also observed that electrodes wear preferentially fasteron the preionization tube side of a fluorine gas discharge laserchamber. In addition, the electrode ends tend to wear at a higher ratethan the bulk of the electrodes, e.g., from within about 3 inches from apoint at the end of the electrode at either end of the electrodes wheregas discharge ceases to occur between the cathode and anode. Therefore,to avoid end effects, applicants only measured electrode erosionstarting at 4 inches from each electrode end, across the entireelectrode longitudinal length between those points. Erosion data for allelectrode segments and samples was then compared based on millimeters oferosion per billion pulses (“Bp”). FIGS. 1-3, below, illustratenon-uniformity in electrode wear. FIG. 1 shows anode profile changesover Bp. FIG. 2 shows cathode profile changes. FIG. 3 shows a typicalaxial anode erosion profile for a 4 KHz ArF laser over approximately 3Bp.

Anode and cathode erosion including discharge width data for a segmentedcathode are shown in FIGS. 4 and 5. FIG. 4 shows local C26000 cathodeand anode erosion rates in mm/Bp for a 2.5 KHz ArF laser operating at100 V C₀ for approximately 2.3 Bp. FIG. 5 shows cathode discharge widthchange for different materials for a 2.5 KHZ ArF gas discharge laseroperating at 1100 V C₀ for approximately 2.3 Bp. One can see that pureCu alloys appear to wear more slowly than Zn containing alloys.Pb-containing brass alloys, e.g., 6 and 7 in Table 1, and high Znalloys, e.g., 5 in Table 1, also demonstrate more discharge widening(ArF) during the same relative number of pulses. In general, lowestcathode wear rates were observed with low Zn alloys (0-30% Zn). Pbcontaining cathode alloys were observed to wear faster in ArF laserdischarges.

FIG. 6 shows surface roughness vs. alloy type measured by “Pocket Surf,”which is a mechanical profilometer. The measurement was afterapproximately 2.3 Bp in a 2.5 KHz ArF laser at 1100 V. Regarding surfaceroughness changes, cathode surface pitting appears to increase anoderoughness and may accelerate the anode erosion rate. The anode, e.g.,with the alloy No. 4 from Table 1, in general, appears smooth incomparison to all of the cathode samples after 2 Bp. Anode roughness isless than 20 micro inches roughness average (“Ra”), in comparison tocathode roughness (90 to 180 micro-inches Ra). Ni produced the smoothestcathode and adjacent anode surface after erosion. Applicants believethis is related to Ni being a single phase material with a high meltingtemperature and forming a chemically weak fluoride. Ni does wearrelatively fast as a cathode, however Ni reduces opposite anode erosionby 50% when compared to brass alloys. This may be due, however, to otherthan the composition of the Ni, e.g., the surface roughness of the Niand relatively low opposite alloy 4 from table 1 anode erosion couldalso suggest that anode erosion is accelerated by cathode rougheningwith or without a Ni or mostly Ni cathode.

Electrode erosion rates for the 2 KHz segmented anode and cathode testswere generally in line with erosion rates measured from electrodestested in 4 KHz laser chambers. It appears that for ArF, repetition rateor duty cycle changes do not significantly change the anode or cathodewear mechanism. Comparison cathode erosion data for the other 4segmented anode tests is summarized below. Applicants tested alloys 1and 2 from Table 1 for cathodes apposite the segmented anodes. Cathodeerosion rates were similar to those measured in the segmented cathodetest presented above. In addition, low Zn containing cathodes could pit,potentially increasing anode wear rates. Pitting was far worse oppositeself-passivating anode samples. It appears that the anode passivatingcoating (reef-like coating) accelerates cathode wear in Arf discharges.Applicants do not fully understand the mechanisms for this, since thereef changes both electrode roughness and surface electrical impedance.In general, cathodes in ArF chambers wear at 0.03-0.05 mm per billionpulses when the anode is noted. Applicants' assignee had been previouslyaware of a 9 Bp 4 Khz ArF laser chamber that demonstrated alloy 4 (fromTable 1) cathode wear rate of 0.03-0.04 mm/Bp when run opposite an alloy8 anode. Applicants also discovered that KrF cathodes tend to wear muchfaster than ArF cathodes, approximately 0.05-0.06 mm/Bp. This is about a20-30% higher erosion rate for KrF cathodes vs. ArF cathodes. Inaddition, cathode wear rates are accelerated much more in ArF forreefing (self-passivating) anodes. Reefing anodes appear to damage ArFcathodes much more so than KrF cathodes made out of Cu—Zn alloys.Average cathode erosion data for the 4 segmented anode chambers can beviewed in FIG. 7.

The presence of an anode reef appears to increase cathode erosion incorresponding regions for both ArF and KrF applications. Applicantsbelieve that the reason is that the reef causes spatially non-uniformelectric field and current density. This may be mitigated by engineeringmore uniform artificially created reefs as opposed to naturally createdreefs, as discussed in more detail below. For KrF reefing systems,cathode erosion rates are generally increased by 10 to 20% when comparedto a non-reefing system. For ArF, the reef impact on cathode erosionrates appears to be even greater, where cathode erosion rates areincreased by 30 to 45%. Increasing cathode erosion rates, however, maybe acceptable if the anode does not wear and the self-passivatingcoating does not impact laser performance, which, again, may be thesubject of so-called reef-engineering, as discussed below.

Cathode erosion in the KrF chambers is found to be higher than that inArF chambers. This is believed to be due to the higher gas operatingpressure in ArF chambers and/or the fact that Argon ions are lighterthan Krypton ions, both of which lead to lower sputtering yields.

There appears also to be an interaction between the anode and thecathode. The magnitude of anode erosion appears to depend not only onthe anode material composition itself (how it reacts with fluorine), butalso on cathode surface morphology changes during erosion. Anodes appearto wear more slowly when facing a cathode material that erodes smoothly.The problem for both ArF and KrF laser chambers seems to be that it isnot apparent what materials meet this criteria of slow wear, i.e., moreslowly than the presently used brass alloys as a laser cathode. Nickelwears fast and smoothly as a cathode. Segmented cathode tests havedemonstrated that the C26000 anode segment opposite this Ni cathode woreat 50% the rate of adjacent segments facing brass alloys. Anoderoughness appears to be about the same for most segments. FIG. 8 showsworn anode and cathode surface morphology v. the composition of thebrass for a C26000 anode in an ArF laser operated at 2.5 KHz forapproximately 2.3 Bp.

Optimizing electrode life requires selection of materials thatphysically wear as slowly as possible, e.g., so as to prevent dischargegap widening, and that remain relatively smooth and homogeneous. For ArFlaser chambers, applicants had been unsuccessful to date using naturallygrowing self-passivating alloys since they suffer from high rep rateperformance issues, e.g., they act as insulators reducing the dischargeenergy locally and overall. Therefore, for ArF, the anode was believedto be required to be non-self-passivating and the cathode to wear asslowly as possible without pitting or roughening. From applicants'testing, Ni alloy cathodes produced the slowest C26000 anode wear rates,while pure Cu alloys produced the lowest cathode wear rate. However, Niwears fast as a cathode alloy, therefore seeming to disqualifying it foran ArF chamber application. In a similar sense, using pure Cu alloycathodes accelerates anode erosion due to pitting. As one increases thealloy Zn content to reduce pitting, the wear rate of the cathodeincreases. Therefore, cathode optimization would seem to require anengineering tradeoff considering surface roughening of the cathode vs.absolute wear rate.

For Cu—Zn alloys, it is apparent that, as Zn content is increased,cathodes wear faster. However, for Cu—Zn cathodes, the cathode wear rateis very slow compared to the anode wear rate. Though high cathode Znlevels increase erosion rates, there is still about a difference of 0.01mm/Bp (20%) over a 40% range in Zn content. Therefore, using a 20-30% Znalloy can yield a good combination of low cathode wear rate and anode“friendliness.” FIG. 8 shows cathode wear rate in 2 KHz ArF laserchambers vs. Zn content. Pictures demonstrating cathode morphology areattached to the figure, helping demonstrate that some Zn is needed tosuppress local non-uniform melting and pitting on the cathode surfacewhen naturally occurring reefing is the operative mechanism. Applicantsestimate from surface chemistry changes measured by SEM and XRF, that upto 15% of the surface Zn can be extracted by the discharge prior toablation of the material.

Cathode related surface chemistry changes following exposure to thelaser chamber discharge are illustrated in FIG. 9 for a 2 KHz ArF laserat up to about 2.3 Bp. In general, applicants have observed that thecathode region exposed to the discharge is depleted of Zn by up to 15%absolute. In addition to having a higher sputter yield than Cu,applicants believe that the Zn may also vaporize at cathode operationaltemperatures. Cathode erosion results in local Zn depletion of the brassin the discharge footprint region. Zn has a relatively high vaporpressure for temperatures >500 C. Therefore applicants suspect weightloss due to Zn vaporization also, e.g., when cathode surface is >500° C.Visible melting of Cu and the presence of non-equilibrium PbF₄ deposits,as seen by XRD Analysis, on KrF cathodes (PbF₄ only being stable between300° C. and 800° C.) suggests to applicants that a cathode surfacetemp >500° C. is realized in operation.

It may be difficult to decouple sputter and vapor pressure effects,since they would also suggest that Zn is preferentially removed from thebrass. Regardless of what mechanism, it appears to applicants that thereis a need for Zn at the cathode surface to suppress cathode pitting.Since cathode wear morphology appears to stabilize with Zn levelsgreater than 20%, applicants suggest using brass alloys with greaterthan 25% Zn as cathodes is one approach for optimum electrode life.Anode surface chemistry was observed to match the bulk electrodechemistry. Since the anode wears much faster than the cathode (alsoapparently coming from chemical attack) this suggests that Zn depletionon the cathode surface requires some time to develop. Assuming this tobe the case, a strong argument can be made that thermally induced Znevaporation may be occurring at the cathode surface.

In general, brass alloys outperform pure metals as chamber cathodes. Itis not completely clear why brass erodes so slowly as a cathode influorine gas discharge laser chambers, however applicants believe thatthe reactivity of Zn with F₂ gas (forming ZnF₂ solids) may help slowcathode erosion. Theoretically, the ideal cathode material would consistof: 1) a single compositional element, 2) a single crystalline phase, 3)a low sputter yield element, 4) a large grain size, 5) a low vaporpressure, 6) low metallic and dissolved gas impurities, 7) thegeneration of solid reaction bi-products when corroded by ionizedfluorine gas.

As metals go, Cu and Ni appear to almost fit this profile, howeverneither forms an entirely successful cathode. Pure Cu appears to pit,and consequently not be anode “friendly,” and Ni wears relatively veryfast, though both also have alloys that demonstrate some of the abovepositive traits of a cathode. As predicted, Cu wears slowly as acathode, however pitting appears to be tied to Zn content. Therefore,pure copper is not a reasonable cathode material since it increasesanode wear rates. Zn additions to Cu (brass alloys) reduce the meltingtemperature of the alloy, perhaps allowing some surface material reflowto cover up pit-like defects. Zn is very reactive with Fluorine, andtherefore it may also play a more chemical role in cathode pitsuppression. Ni, on the other hand, wears smoothly as predicted (i.e.,is anode “friendly”), but faster than is desirable. This may be due tothe poor reactivity of Ni with Fluorine gas. Ni does not pit as acathode material, suggesting that cathode pitting is not related tosurface chemistry effects (i.e., Zn is not needed to suppress pitting).Perhaps it may not be possible to find a material with all of the abovetraits. Brass is inexpensive, and wears at a consistent and predictablerate as a cathode, and appears not to pit in the case where the contentof the Zn is >20% (percent by weight).

Brass cathode pitting can be tuned by adjusting alloy Zn content.Applicants suggest that Zn additions should be held to a minimum sincecathode erosion is accelerated if high Zn levels are used. Perhaps forself passivating alloys, using an alloy 5 cathode may resist cathodepitting damage better than alloy 4. In addition, Pb additions to brasscathode alloys appear to increase wear rates and cathode roughening. Pbis not uniformly distributed in brass, and has the unusual ability toprevent grain growth. The Pb is relatively uniformly distributed in thebulk brass alloy, however, by non-uniformity is meant that, since, e.g.,the Pb is not very soluble in the alloy it segregates and “clumps” atgrain boundaries in different regions, so that, at, e.g., 500×magnification it can be seen that the distribution is very non-uniform.Pb is also highly reactive with fluorine. Pb may accelerate cathodeerosion by increasing fluorine ion surface affinity to the brass (i.e.,may be struck during ringing in the discharge). Pb also pins metalgrains, therefore preserving a fine grain structure. As sputter targets,fine grain materials erode faster than large grain materials since it iseasier to eject atoms along grain boundaries (i.e., reduce surfaceenergy). Since there are 2 possible mechanisms as to why Pb containingbrass alloys wear faster as a cathode than non-Pb containing alloys,applicants suggest using lead free brass alloys as a long life cathode(e.g., using alloys 4 or 5 from Table 1 or a C27000 brass).

It is also possible that, using the above principles to find cathodematerials with various wear rates, one can combine 2 different alloyswith differing erosion rates to create differential erosion cathodes. Adifferential erosion cathode is illustrated in FIG. 10. Applicants havetried both Cu10200/alloy 4 and alloy 4/alloy 8 diffusion bondedcathodes. Both systems appear capable of exhibiting differential erosionin KHz or ArF chambers.

FIG. 10 shows an electrode system containing a cathode 20 having a mainbody 21 made of a first material and an insert 21 made of a secondmaterial, with the first material 21 having a higher erosion rate thanthe second material 24, thus eventually forming a differential erosiontrench 24. It will be seen that the insert 24 eventual erodes away fromthe anode 22 over time as well. FIG. 10 also shows a cathode having amain body 23 made of a first material and an insert 26 made of a secondmaterial, e.g., one that will form a passivating layer (reef) 28 as ananode in a fluorine gas discharge laser. The differential erosion ratesof the first and second materials, aided by the reef 28, eventuallyforms an erosion trench 27 on either side of the insert 25 in the anode22.

A Cu10200/alloy 4 cathode is illustrated in FIG. 11, which has beenexposed to >2.5 Bp, at 4 KHz in an ArF laser chamber. One can see that astep 26 has formed on the PI tube side of the cathode, the left side asshown in FIG. 11, confirming that the alloy 4 is wearing faster than theC10200 (as predicted).

FIG. 12 illustrates an alloy 4/alloy 8 diffusion bond (Ni interlayervisible). From FIG. 12, one can see the large difference inmicrostructure between the annealed alloy 4 and alloy 8. Diffusion bondstrength for the Cu10200/Alloy 4 process was approximately 5000 PSI. Thesupplier Tosoh SMD was able to achieve bond tensile strengths of greaterthan 16,000 PSI (40-50% Yield strength) for the alloy 4/alloy 8 system,e.g., due to using a metal intermediate layer, e.g., the Ni layer shownin FIG. 12. Applicants believe that this higher bond strength will beadequate for a chamber electrodes, e.g., cathode, applications asexplained further below.

While diffusion bonding is common in some other industries, e.g., thephysical vapor deposition (“PVD”), i.e., sputtering, target industry,applicants are not aware that it has been used for the formation ofmulti-material, or the like, electrodes, and specifically not forfluorine gas discharge electrodes, and further specifically not forbi-metallic electrodes that better maintain shape over life and/or fordifferentially eroding structures. More specifically, applicants are notaware that such alloys as alloys of brass, e.g., C36000 and C26000 hadbeen diffusion bonded before applicants suggested this be done, and morespecifically be done for electrodes, and specifically for electrodes ina fluorine gas discharge laser environment.

Applicants company approached the above referenced vendor, Tosho SMD, ofGrove City, Ohio to fabricate electrodes of the type variouslyreferenced in the present application utilizing diffusion bonding. Thevendor first created several prototypes, first using a typical Al to Tidiffusion bonding process. The vendor proposed utilizing a then knowntechnique for increasing tensile strength in the bonded materials whileotherwise preserving and even enhancing the structure of the bondedmaterial and also preserving the required alloy microstructures of thebonded materials. The vendor proposed forming an intermediate adhesionmetal layer between the metal being bonded. This layer may be formed,e.g., by coating each of the materials to be bonded at the bondinterface, which are thereafter subjected to the diffusion bondingprocess. The surfaces of the respective materials at the diffusion bondinterface may be roughened prior to such coating and the coating may bedone by a number of thin film or thick film coating techniques well knowin the art. This process is quite common for joining materials, asdiscussed in T. Nguyen, “Diffusion bonding—an advanced material processfor aerospace technology”, Aerojet, Sacramento, Calif.,http://www.vacets.org/vtic97/ttnguyen.htm.

The resultant bimetallic electrode exhibits bond strengths over otherwell known bonding techniques on the order of some 4×, while at the sametime preserving required alloy microstructures and other propertiesessential to proper performance as electrodes in a fluorine gasdischarge laser environment. The vendor proposed a Ni adhesion layercoating, which has proved particularly successful in achieving resultsthat are desirable, e.g., for electrode applications for fluorine gasdischarge lasers. Applicants also believe that this technique of bondingcan be employed in making electrodes for the next generation laserlithography light sources, e.g., EUV electrodes, e.g., as platform or,e.g., Cu or Ni to which is bonded a hollow, e.g., generally cylindricalWolfram (“W”) or Wolfram-Thorium alloy (“W-Th”) EUV source, e.g., for aplasma focus source.

This has proven effective for the manufacture of multi-metallicstructures of different brass alloys and relatively pure Cu and a brassalloy.

FIG. 13 shows a prior art example of an anode 80, having an anode blade82, made of a metal in roughly a blunted blade shape, having a frontside (in the direction from which laser gas is moved past the anode 80)with a front side fairing 84 and a rear side fairing 86. Each of thefront and rear side fairings is made, e.g., of a ceramic insulator. Theentire assembly is mounted on an anode mount 88.

Based on electrode erosion data, one can determine the etch rate ratiosbetween the lower wearing center element and the high wear rate outermaterial in a differential erosion cathode. In general, one would liketo select a material for the outer part of the cathode that wears fasterthan the center. Care also must be taken to select a center materialthat does not pit the cathode. Applicants have found that edge to center(“E:C”) erosion ration (so-called “selectivity” of erosion) etch rateratio of 1.17 is able to develop into a differential erosion cathode,i.e., the material in the center eroding more slowly. One can do betterwith Copper based alloys, however, by combining alloy 4 from Table 1 andalloy 8 from Table 1 an E:C etch rate ratio of 1.6 attainable. Theoptimum, anode “friendly,” combination of materials would seem to beNi201 and C26000, with an E:C ratio of >5.8. It is not clear what theoptimum ratio is for E:C etch rates, however at this time applicantsfeel that the higher the ratio, the better. One possible choice iscombining C26000 and C36000 for a low risk differential erosion cathode.TABLE 2 Erosion Rate Ratios for Proposed Differential-Erosion CathodesDifferential Erosion Cathodes Center Edge Selectivity (E:C) C10200C26000 1.175438596 Demonstrated to 2.6 Bp. Known to pit C26000 C360001.625 Tested >11 Bp C22000 C36000 1.857142857 Ideal full brass (may pit)C26000 Ni201 5.875 Best, Safe C22000 Ni201 6.714285714 Best overall

From the above, applicants have concluded that Cu—Zn cathode alloyserode at about 0.5 mm per billion pulses in ArF chambers, ArF anodematerials (C26000 typical) typically erode at 0.15 mm/Bp. Cathodeerosion rates in KrF chambers are about 20% higher than ArF chambers forsome materials. Cathode wear rates seem to scale linearly with chamberrepetition rates. Zn additions to Cu suppress cathode pitting whileslightly increasing cathode erosion rates. Ni alloys are very anode“friendly,” however wear much faster than Cu alloys as a chambercathode. Anode roughening or reef (self-passivation) formationaccelerates cathode wear. Pb addition can increase cathode wear rates,and is not recommended for long life cathode centers. Cathode pittingappears to increase anode wear rates. Alloy 4 from Table 1 is a goodoverall cathode material for ArF and potentially KrF chambers.Differential erosion cathodes appear to work well for both KrF and ArFapplications. Differential erosion cathodes appear to work well withsystems with E:C etch rate ratios >1.15. Differential erosion cathodeswill likely work well for the long life KrF chamber applications, withproper alloy selection. In general, using alloy 4 instead of alloy 8 issuggested for improving KrF cathode life.

The present application contemplates a number of utilizations of alloyand composite alloy uses for electrodes and specifically fordifferential erosion rate materials, to control, e.g., electrode shapesafter wear, rates of wear, etc. These include the following. Applicants'propose the use of high Zn, high Zn alloys, pure aluminum or high Snalloys, in conjunction with, e.g., bonded, e.g., by diffusion bonding,to high Cu alloys, e.g., with greater than 80 percent Cu. The formerhigh Zn, Zn alloy, Al and high Sn alloys erode relatively quickly andform with the higher Cu alloys preselected and beneficial wear over thelife of the electrode. It will be understood that these and likedifferential wear patterns discussed in the application can beself-regulating. That is, as the higher erosion rate material, e.g., oneither side of a lower erosion rate material erodes to form a depressionon one or both sides of the lower erosion rate material, the dischargewill become more focused on the protruding lower erosion rate material,which will then erode at a faster rate until erosion in the highererosion rate material on the side or sides of the central portion beginsto differentially erode again.

The present application contemplates the use of a cladding process toclad pure Cu to a self passivating alloy. Applicants have determinedthat copper, e.g., pure copper will wear faster than anyself-passivating electrode.

It is also possible, as is shown in FIG. 15 to create a shallow trench50, e.g., of about 0.5 inches in maximum depth, on either side of, e.g.,the crown 52 of an electrode 54, which may, e.g., be of the samematerial as the rest of the body of the electrode 54, and to fill thetrench 50, e.g., with, e.g., zinc, e.g., in the form of zinc alloy orpure zinc, e.g., in molten form. Thereafter, after the zinc/zinc alloycools the assembly can be machined to form the desired shape, e.g., withthe crown 52 similar to the blade 82 in FIG. 13, with the trench 52 andremainder of the electrode 54 replacing the fairings 84.86 shown in FIG.13. Alternatively there may be formed the cross-section of FIG. 11, withthe crown 52 in the position of the insert shown in FIG. 11. Indium oralloys or pure indium can similarly be used in replacement for the zincfor making anodes.

It is also possible to leave the just-mentioned trenches empty tocontain the discharge on the portion of the electrode on the crown 52between the trenches. This could also be done with multiple trenchesflanking respective multiple extending discharge receptor protrusions inthe nature of multiple crowns 52. Materials, e.g., spray coatings orelectroless coatings of, e.g., pure Ni, Ni alloys, e.g., Ni—Cu alloys,Sn or Zn or their alloys may also be deposited in such trenches 50,e.g., by spraying, thus flanking the electrode crown 52, and thenmachining the whole into a final shape. A Damacene process may be usedin this instance. A Damacene process, e.g., may involve the depositionof a material on a relatively rough surface and the polishing down ofthe high points to give an inlaid finish.

Another material that can be used to flank, e.g., a discharge crowncould be gildcop, an alumina doped copper material useable as a higherosion rate material.

As shown in FIG. 19, applicants have also developed another form of whatmight be called a “solder sidewalk” or alloy sidewalk cathode in whichthe trenches 50 as shown in FIG. 15 may be replaced with rotated “V”shaped slots 60 in which the one side 60′ of the “V” lies in essentiallythe vertical plane and the other side 60″ lies at an angle to thehorizontal. The respective one sides 60′ of the respective “V”'s 60together form the side walls of a crown portion 62 of the electrode,e.g., a cathode 64. The rotated “V”'s may be filled with a suitablefaster eroding material than that of the cathode 64, e.g., a materialcontaining Pb, e.g., a Pb solder, while the cathode 64 may be made,e.g., of oxygen free copper (“OFC”) copper, e.g., annealed to obtain amaximum or nearly maximum grain size, as such annealing is known in theart. The “sidewalks” formed by the “V”'s may be formed by placing thematerial for the sidewalks, e.g., the Pb solder in the rotated “V”'s inmolten form or melting it into the “V”'s, allowing it to harden and thenmachining it down to the contour of the cathode 64.

In the course of fabricating some of the electrodes discussed in thepresent application above various elements could be, e.g., fabricatedseparately and joined by various means, including, e.g., bolting,cladding or bonding, e.g., diffusion bonding, and perhaps then could bemachined or further machined to form the desired end product shape.Bonding processes may include various ways of forming compositematerials, e.g., single piece electrodes formed by diffusion bonding,explosion bonding, cladding, ultrasonic welding, friction welding,galvanizing and the like.

An upper surface of an electrode containing high content of a particularmaterial, e.g., Zn, may be formed, e.g., by annealing, e.g., certainhigh Zn content brasses, e.g., alloy 4, C27000 or alloy 8, e.g., attemperatures of 1200° F., e.g., for 60 minutes, e.g., per inch ofthickness. Applicants have found that thereby relatively thick layers ofrelatively pure Zn, e.g., of between 0.005 and 0.010 inches are formed.Applicants refer to this as, e.g., a Zn oxidation process, whichsegregates the Zn to the surface of the zinc-containing alloy. Zn,having a high affinity for oxygen readily segregates to the surfaceforming ZnO₂ at the surface.

One form of passivation layer/reefing engineering that applicants havediscovered makes use of processes such as those utilized in themanufacture of tin nanowires using a vacuum infiltrated porousanodization of aluminum. Such techniques can enable the creation of asynthetically created reef layer made of, e.g. a metal fluoride, on anelectrode created with relatively controlled pore size and distribution,having such properties as a controlled and evenly distributed porosity,e.g., as an anode reefing layer. In this manner the barrier reef may beengineered for controlled chemical and electrical properties, e.g.,impedance, e.g., to control the surface charge on the reef to, e.g.,avoid arcing, and in this manner optimize corrosion/erosion resistanceand optimize impedance.

Utilizing such a technique can enable the growing of self-passivationmaterial in relatively columnar and relatively evenly distributedstructures, resembling, e.g., in one embodiment an orchard of treetrunks and in another a honeycomb with generally circular openings and asurrounding latticework structure. This relatively evenly distributedorchard of tree trunks/laticework develops naturally through themechanism of the anodization process and the resultant treetrunks/laticework can be employed to limit the transmission of, e.g.,charged ions present in the laser gas discharge, through the openings tothe electrode surface while maintaining the electrical conductivity,though the “tree trunks” themselves, as oxides, are insulative. Thissame mechanism of blocking ion transport can be employed in theanodization process as to the reactants to control the growth andseparation of the “tree trunks”/holes in the latticework.

The uniformity of the growth and separation of the “tree trunks” may beenhanced and its control made more uniform by, e.g., pre-texturing thealloy surface on which the artificially engineered reef is to be grown.This can be done, e.g., as shown in FIGS. 14 a-f. In FIG. 14 a there isshown a substrate 30, which may be the surface of an electrode. Thesurface 30 of the electrode is shown in FIG. 14 b to have been grown afirst anodization 32, which contains a plurality of relatively evenlydistributed “tree trunks” 34 and pores 36.

In FIG. 14 b there is shown that after removal of the first anodization32 the surface of 30 is left textured. This step is then followed, asshown in FIG. 14 c, with a second anodization 32′, which as is shownresults in a more uniformly distributed set of tree trunks 34′ and pores36,′ and also with the vertical orientation of the pores 36′ being moreuniform. The formulation in FIG. 14 c may be useful in its own right asan artificial reef coating on the surface of, e.g., an electrode. It mayalso be further enhanced for desired results by, e.g., as shown in FIG.14 d, widening, e.g., the pores, e.g., by an etching process. This mayalso be followed by or include, as shown in FIG. 14 e a thinning orremoval of the barrier layer 40 at the bottom of the pores 36′ andsubsequently filling the pores 36′ with a conductive material, e.g.,metal 42. This process may be implemented in-situ in an operating laserchamber, e.g., by reversing electrode polarities in the step of removingthe first reef.

Oxides other than aluminum oxide, e.g., ZnO, SnO₂ and PbO, which aresemiconductors in nature, but mostly insulative and have a relativelylower sputtering erosion rate than pure metals or metal alloys can beused, e.g., on the anode, to resist fluorine attack, without alsoadversely impacting impedance. This is proposed by applicants as asubstitute to the naturally growing reefing, which applicants have foundcan have adverse impacts on, e.g., system impedance, e.g., at higher gasdischarge pulse repetition rates, e.g., over 2000 Hz.

As opposed to forming an insulator and metallically doping the layer ormechanically or ionically drilling holes in the layer on, e.g., ananode, applicants propose to grown an oxide layer over the surface of anelectrode formed of the material which also forms the oxide, in pureform or alloy form, to create the oxide layer. Some such materials inthe pure form may not be suitable for use as an electrode, e.g., pure Znor Pb, due to other reasons, e.g., mechanical strength. However, forsome alloys, e.g., CuZn, e.g., subjected to oxidation in, e.g., afurnace oxidation process, an ozone oxidation process, a plasmaimmersion ion implantation oxidation process, an O₂ plasma surfacetreatment oxidation process or by annealing in an oxygen atmosphere, orthe like process, the ZnO will grow faster than the CuO or CuO₂. The ZnOis also a more stable molecule. Also, due to the thin film nature of thegrowth, the layer of ZnO and CuO or CuO₂ will be very dense and protectthe electrode surface from erosion, e.g., due to fluorine attack, e.g.,due to sputtering under fluorine ion bombardment, and at the same timenot impact impedance significantly. Normally such a thin film oxide,e.g. SiO₂ is an insulator, e.g., in integrated circuit manufacture, butnot at the thickness involved in the present application and at thevoltages across the electrodes in a gas discharge laser as discussed inthis application.

Surface texturing may also be applied by, e.g., grit blasting theelectrode surface. This may be, e.g., a substitute for the firstanodization step shown in FIGS. 14 a and b.

Differential erosion electrodes, containing, e.g., low erosion materialon the crown of the electrode, generally where the discharge region willmostly be, and higher wear rate material flanking the lower wear ratematerial on the crown, in order to maintain an electrode shape profilein the discharge region may be formed, as has been demonstrated byapplicants in, e.g., 2 KHz ArF and KrF laser chambers, considerations ofsuch other properties of the electrodes, e.g., thermal properties, e.g.,heat transfer coefficients and profiles may also be taken into account.

Diffusion bonding and other ways to avoid, e.g., solder joints may alsoattribute to the proper and adequate fabrication of such bimetallic ortri-metallic electrode bodies, achieving, e.g., with diffusion bondingmulti-metallic structures that are formed with dense, high strengthbonds with excellent thermal conductivity. The bonding process, e.g.,diffusion bonding, e.g., utilizing an intermediate very thin thirdmetal, e.g., Ni, layer, as shown in FIGS. 11 and 12 can be utilized toengineer the profile of the multi-metallic electrode structure to alterthermal and/or chemical properties of the electrode body. As shown,e.g., in FIGS. 16 a-d, electrodes may be engineered to, e.g., modify theheat transfer through the electrode, e.g., to its electrode mount, inorder to, e.g., modify the thermal interaction between the electrode andits electrode mount, e.g., to prevent cracking of the mount, which mayoften be a relatively brittle ceramic material subject to its owninterior thermal stresses and to the, e.g., bending of the metalelectrode induced by temperature changes and/or of the gas dischargelaser chamber under pressure.

FIG. 16 a shows a relatively “hot” electrode 70, e.g., a cathode, whichis formed from a body 72 comprising a material, e.g., brass, with arelatively low thermal conductivity and a relatively low meltingtemperature, diffusion bonded with an insert 74, of a material, e.g.,Cu, having a relatively high thermal conductivity and a relatively highmelting point. FIG. 16 b shows a “medium temperature electrode 70′,e.g., a cathode, comprising a body 72 and an insert 74, forming thecrown of the electrode, along with an additional insert 76. The body 72may be formed of a material with a relatively low thermal conductivityand melting point and the inserts 74, 76 may be formed of a materialhaving a relatively high thermal conductivity. The second insert 76 mayserve to carry more thermal energy away from the insert 74 and the body72 of the electrode and to focus the thermal energy flux more at theboundary of the insert 76 and the electrode mount (not shown). FIG. 16 cshows a relatively “cold” electrode 70″ in which the expanse of thesecond insert 76 is such as to leave only a shell of the electrode body72 formed of the low thermal conductivity and low melting point materialand to leave much of the cross-section of the electrode 70″ formed ofthe relatively higher thermal conductivity and melting point material76. FIG. 16 d shows another embodiment of the “cold” electrode 70′″ inwhich another insert 78 is contained within the insert 76 as shown inFIG. 16 c composed of a material that is of a relatively low thermalconductivity and relatively low melting point, e.g., a brass, which mayor may not be the same as the brass in the portion 72 of the electrode70 body, and may be positioned to engineer the thermal interactionbetween the electrode 70′″ and, e.g., current feed through/mountingbolts, which may be attached to the electrode 70′″ along itslongitudinal extension in the region of the additional insert 78.

In the manner described above, the thermal profile of an electrodes70-70′″ and, e.g., its interaction with such elements as its ceramicmount and metallic feed-throughs, along with the thermal interaction ofthe electrode and feed through assembly and the ceramic mount/insulatormay be better engineered. It will be understood that variousutilizations of the bonding techniques discussed in this application,e.g., diffusion bonding, facilitates the fabrication of thesemulti-segmented electrodes and, as discussed, tuning their thermalproperties. However, this particular embodiment f the present inventionmay also utilize simpler mechanical bonding, e.g., bolting or screwingand achieve the benefits of the embodiment of the invention.

FIG. 18 shows a cathode manufactured utilizing diffusion bondingtechniques that provides for longer cathode life without modification ofexisting cathode size, shape or other geometries. FIG. 18 illustrates ablock of material 120 which may be formed of a bar 122 of relativelyvery low erosion rate material, e.g., pure or essentially pure copper,e.g., annealed at 1200° F., forming a material having, e.g., a very lowsputter rate material in a fluorine gas discharge laser environment.This bar 122 may be, e.g., bonded, e.g., by diffusion bonding, to a bar124 of, e.g., brass, e.g., C26000 or C36000 or the like brass alloys,e.g., annealed at 700° F., providing, e.g., good machining andmechanical properties. The latter may be of particular importance inregard to the attachments and sealing requirements between the cathode120 and, e.g., the main insulator in a fluorine gas discharge laser.

The two bars 122, 124 may then be bonded together by a variety ofbonding techniques as noted above, giving, essentially molecular typebonds, e.g., diffusion bonding, as described above, and which may alsobe, e.g., further enhanced by putting a thin layer of bonding catalyst,e.g., a metal adhesion layer, e.g., Ni to facilitate an even smootherand tighter diffusion bond 126.

Thereafter the electrode may be machined from the bonded bars, e.g., asthe outline 128 suggests.

In this manner a bimetallic cathode can be created that structurally,thermally, mechanically and in other ways behaves as if it weremonolithic, but the upper portion is very wear resistant in the fluorinegas discharge laser environment and the lover portion has manybeneficial properties that a pure copper electrode would not possess.This gives a very low erosion rate electrode, e.g., a cathode, withoutthe need for reliance on differential erosion, e.g., with an insert orcrown at the discharge region.

Applicants, through observation of the behaviors of anodized aluminumcorrosion in fluorine gas discharge laser environments have proposed tocombine anodized aluminum with, e.g., fastback fairing technology to,among other things, reduce downstream arching. An example of this can beseen in FIG. 17. FIG. 17 shows a cross-sectional view of an electrodearrangement 90, transverse to the elongated extension of the electrodepair 90, the electrode pair 90 comprising, e.g., a pair of gas dischargeelectrodes comprising a cathode 92 and an anode 94.

The anode 94 may comprise, e.g., a front-side (in the direction fromwhich laser gas is moved between the electrode pair 90) including afront side fairing 96 having an upper front-side fairing longitudinallyextending surface 98, extending longitudinally along the length of theanode 94. The apposing front side fairing bottom surface 100 rests,e.g., on an anode mount (not shown), attached to the interior walls ofthe chamber (not shown). The anode 94 may also comprise, e.g., a backside wall 102 and a front side extension 104, extending from the uppertermination of the front side fairing 98. Extending between the upperextension of the front side extension 104 and the upper extension of therear side wall 102 may be formed, e.g., a discharge region 106 of theanode 94. The discharge region 106 opposes generally the dischargeregion of the cathode 92, e.g., generally along the longitudinal axis ofthe cathode 92.

Abutting the rear side wall 102 may be, e.g., a rear side (downstream ofthe gas discharge laser gas flow between the electrode pair 90) having,e.g., a rear side fairing 110 having a rear side fairing upper surface112, which may extent slightly higher along the rear side wall 102 thanthe intersection of the front side fairing 96 and the front sideextension 104. The rear side fairing 110 may, therefore, extendlaterally a longer distance than the front side fairing 96. The rearside fairing bottom surface 114 may also rest on the anode mount (notshown).

The entire anode 94 may comprise, e.g., Al. The exposed surfaces of theanode 94, including, e.g., the front side fairing upper surface 98, thefront side extension 104, the discharge region 106 and the portion ofthe rear side wall 102 extending above the abutting rear side fairing110 may be coated with anodization of the material of the anode 94,e.g., anodized aluminum, e.g., from about 0.5-2.0 mils thick. Thethickness may be selected (tuned) for the desired combination ofimpedance and erosion resistance. The surface may also be roughened,e.g., with a 30-80 micro-inch Ra abrasive Al blast, e.g., air propelledsand. Grit-blasting/roughening could also be accomplished with anon-silica containing media. The anode may also be fabricated as notedabove utilizing, e.g., an Al 1100-0 alloy of 99% commercial gradealuminum along with other Si free aluminum alloys.

The above-described anode may be especially useful in promoting longlife in fluorine gas discharge lasers, particularly ArF fluorine gasdischarge lasers. This is particularly useful in applying thecombination of a blade dielectric and a monolithic anodized anode 94with a monolithic front side fairing 98, which the applicants have namedan “airstream” anode after the aerodynamic aluminum mobile homes of themid-twentieth century. The passivated long life, e.g., anodized, pure oressentially pure aluminum anode with the “airstream” front fairing 98combined with the rear fairing 110 of a known blade dielectric anode,e.g. as generally shown above in FIG. 13, and slightly modifiedaccording to the presently described embodiment, can have a number ofbeneficial effects.

It will be seen that the combination of the blade portion of the anodeexemplified in FIG. 13 with the monolithic front side fairing 96 of theabove described embodiment can serve to make the anode 94 easier tomanufacture. This is due to the fact that pure aluminum or substantiallypure aluminum is a relatively soft material and the larger monolithiccross-section gives the structure more strength to withstand themanufacturing processes, e.g., machining. Also, aluminum anodization isa very controllable process and can be tuned to optimize anode dischargebehavior from an erosion standpoint and at the same time provideadequate impedance characteristics for proper discharge between theanode and the cathode. Thus can be avoided the uncertainties ofnaturally grown anode electrode reefs and the F₂ chemistry problemsfound to be associated with naturally grown reefs. Such tuning providesfor longer life of the anode, e.g., in a fluorine gas discharge laser,e.g., an ArF gas discharge laser. Finally the just described anode isless costly overall to manufacture, and install, including cost ofmaterials and fewer numbers of parts, than, e.g., the blade dielectricanode illustrated, e.g., in FIG. 13. A further advantage is theaerodynamics of the monolithic anode 94—front side fairing 96combination, and the fact that leading edge modifications, e.g., for gasflow dynamics, may readily be machined without having to worry aboutmachining a ceramic material.

Applicants have demonstrated an electrode as described with purealuminum and a sulfuric acid anodization in a fluorine gas dischargelaser, e.g., a KrF fluorine gas discharge laser at up to 2 Bp withoutevident damage.

An electrode system that applicants believe to be beneficial, e.g., inshaping the discharge and maintaining the shape of the discharge overelectrode life comprises, e.g., an electrode system 130 as shown in FIG.20 a. FIG. 20 a shows a cathode 132 and an anode 134, with the anode 134having imbedded therein, below the discharge region a magnet 136, e.g.,a rare earth magnet, e.g. which can be purchased at—, e.g. RadioShackand cathode 132 has imbedded therein below the discharge region asimilar magnet 138. The north and south poles of the magnets 136, 138are oriented oppositely so that a magnetic flux field is formed betweenthe anode magnet 136 and the cathode magnet 138, encompassing the gasdischarge 146 between the cathode 132 and the anode 134. This magneticflux field tends to confine the discharge in the horizontal plane totend to keep it formed more or less directly between the cathode 132 andanode 134 directly over the discharge region of the cathode 132 andanode 134 and to prevent that discharge region from spreading outlaterally over chamber life. The electrons of the gas discharge and ionsas well will tend to be confined within the lines of highest electricand magnetic field, so that the magnetic field boosts the confinementfactor.

FIG. 20 b shows an arrangement where the anode 154 has a pair ofauxiliary magners 160 and 160 distributed on either side of the magnet158, all of which magnets 158, 160, 162 may be rare earth magnets. Thecathode 152 and the anode 154 continue to have the embedded magnets asshown in FIG. 20 a, i.e., magnets 156 and 158, which serve the purposesdescribed above in relation to FIG. 20 a. The auxiliary magnets 160, 162contained in the anode 154 serve to provide a horizontal tuning of theelectric field to go along with the vertical tuning provided by themagnets 136, 138.

FIG. 20 c shows the magnets 136 of the anode distributed generally alongthe longitudinal axis of the anode by way of illustration, with the sameconfiguration being possible for the cathode magnets 138.

Another form of reefing engineer that applicants believe to be of greatbenefit in producing both long lived cathodes and anodes involves theuse of so-called reefing templates. As shown, e.g., in FIG. 21 a anexample of a so-called positive reefing template is illustratedschematically. In a positive reefing templates system, an electrode,e.g., an anode 182 is shown schematically. The anode 182 may be, e.g., amaterial that does not ordinarily form a reef when used as an electrodein a fluorine gas discharge laser, e.g., as an anode in an ArF fluorinegas discharge laser, e.g., a brass alloy of copper, e.g., C36500. Havinga relatively low fraction of Pb, along with a relatively high level ofZn, C36500 does not reef well as an ArF anode, as applicants have foundand as can be seen, e.g., in FIG. 22. C36500 is nominally 60% Cu, 37% Znand 3% Pb. On the upper surface of the anode, as shown in FIG. 21 a 2are deposited in a predetermined pattern a plurality of deposits 184 ofa material that would push the alloy of the anode 182 into the reefingportion of the curve of FIG. 22, had a corresponding reduction in thecontent of Zn occurred, i.e., Pb. It will be understood that manypatterns for the positive reefing template are possible and may result,also along with the materials used, in reefs of differringcharacteristics. For example, the template could for adjoining squaresin a checkerboard fashion, circles surrounded by intervening spaces,polygons surrounded by other polygons with no deposit, squares orrectangles separated by a grid of elongated straight “pathways,” and thelike.

Thereafter, as shown in FIG. 21 a 3 the Pb in the deposits 186 isdiffused under thermal treatment into the upper very thin reaches of thematerial of the anode 182 to form diffusions 184. It will be understoodby those in the art that the diffusion process will tend to altersomewhat the dimensions of the pattern formed by the deposits 184 andthe shapes possibly as well, i.e., the diffusions will spread in aminiaturized version of an ink blot on a piece of relatively porouspaper, which ink does at room temperature and diffusions do under highthermal stress. The remaining shape and dimensions of the diffusions 184will, however, still allow for the reefing engineering according to anembodiment of the present invention.

As shown in FIG. 21 a 4 exposure of the anode 182 to, e.g., a fluorinegas discharge laser environment, e.g., in a ArF laser chamber, eitherduring normal operations or, e.g., burn in testing during manufacturing,will now grow a reef having essentially relatively particularly placedsegments 186 separated by essentially relatively particularly definedspaces 188. The placement of the reef sections 186 in relation to therespective “seed” diffusions 184, and the size of the spaces 188 havebeen illustrated schematically in FIG. 21 a to show that the growth overthe diffusions 184, as was the case with he diffusions 184 themselves,is not entirely uniform in extension away from the respective boundaryof the respective diffusion 184 or on respective opposing sideboundaries of a diffusion 184. Therefore separations 188 betweenneighboring reef regions 186 may not be uniform throughout the reefregion on the anode. Applicants have also discovered that the reefregions 186 so produced are almost free of Pb, so that the Pb diffusions184 catalyze the formation of the fluoride of the reef regions 186,e.g., CuFl, but remain in the substrate of the electrode 182 under thereef regions 186.

Nevertheless, such engineered reefs may be engineered by properselection of deposition templates, i.e., sizes, shapes and patterns ofdepositions 184 and diffusion processes, for somewhat relativelyuniformly forming the diffusions 184, such the reef regions 186 can begrown with their position, shape and extent on the surface of the anode182, e.g., in the discharge region, sufficiently controlled. The extentand positioning of the reef sections 186 and spaces 188 may besufficiently engineered to provide the insulation from, e.g., dischargeand/or ion erosion and at the same time provide impedances in therequired tolerances to promote long lived effective electrodes, e.g.,anodes, e.g., for fluorine gas discharge lasers that are reproducible totolerances that can be effectively manufactured.

A similar process is illustrated schematically in FIG. 21 b 1-4. Here anelectrode, e.g., an anode 190 comprised of, e.g., C36500, has depositedon it deposits 192 of a material that if in greater content in the alloyitself would inhibit reefing as can be seen in FIG. 22, e.g., Zn. The Znis then diffused to form diffusions 194 and then the reef is grownhaving reef sections 196 and openings 198, except that now the openings198 generally conform to the locations of the diffusions 196 as opposedto vice-versa in the process of FIG. 21 a. The same issues ofcontrollability and manageability of the reefing process as were thecase described above for the “positive” reef templating process of FIG.21 a exist. Applicants believe, however, that these are not detrimentalto the production, or perhaps more properly stated for reefs grownduring actual fluorine gas discharge laser operation, the causation, ofreefs with the desired dual characteristics of protection againsterosion and allowing proper discharge, i.e., maintaining sufficientlylow impedance.

Applicants also believe that the size of the grain boundaries and thenumber of grain boundary intersections in the material of the anodeplays a part in this process, e.g., in forming interstices at grainboundary intersections into which the Pb or Zn depositions can migratein the diffusion process. Roughly 20 micron separations of grainboundaries in at least one dimension of the grain seem to promote theprocesses discussed above. In addition applicants have found that aroughly 0.004 inch wide “seed” of, e.g., Pb, can promote the growth of aroughly 0.02 inch wide growth of reef.

Another aspect of electrode life that has been observed by applicantscan be seen in viewing FIGS. 23 a-d. FIG. 23 a shows a schematic sideview of a portion of a gas discharge laser chamber 200 having a cathode202 and a main insulator 204 attached to the top 206 of the chamber 200.During operation of a gas discharge laser the chamber 200 is placedunder relatively high pressure, e.g., of 3-4 atmospheres, and it isknown that this causes to roof 206 to bow upward, taking the maininsulator 204 and electrode (cathode) 202 with it. This is shownschematically and not to proportion, for illustrative purposes, in FIG.23 b. This bowing, while only about 0.005 inches at its peak point, cancreate problems in main insulator 204 cracking, which are addressed inways not the subject of this application, but is believed to also causeelectrode wear problems. Applicants have observed higher electrode weartowards the ends of the electrodes of fluorine gas discharge lasers,e.g., cathodes 202, and the facing portion of the anode 208, which willbe understood may not be at the end of the anode 208, since the anode208 may extend longitudinally longer than the cathode 202. Over time,this causes the discharge to be less compact at this end region and thusless effective, resulting in the overall discharge along the electrodeson average being less effective. This, at least in part, contributes tothe need for raising the discharge voltage required across theelectrodes over electrode/chamber life, in turn causing more erosionalong the remainder of the electrode per unit time as chamber/electrodelive increases. Problems can also result from arcing at the ends of theelectrode with surrounding grounded elements other than the opposinganode, due to the expanded discharge shape, which is also undesirable.

Applicants have proposed a solution to this bowing problem by themachining of a relatively equal and opposite bow in the crown region ofthe electrode, e.g., the anode 208′, as illustrated schematically andout of proportion for illustrative purposes in FIG. 22 c. Thus, when theelectrode 202 and main insulator 204 bow as shown in FIG. 22 b, theresultant profile of the cathode electrode 202 vis-à-vis its gap withthe opposing anode 208, generally not subject to any bowing, will form auniform gap over the entire length of the discharge regions formed onthe cathode 202′ and anode 208′. That is, the discharge region 210 onthe cathode 202,′ while bowed concavely as shown in FIG. 23 b, will nothave its separation from the discharge region 211 of the anode 208′varying significantly over the longitudinal length of the cathode 202and anode 208′ (any more than tolerances would have dictate had thecathode 202 not bowed as a result of the pressure in the chamber 200).Specifically the cathode 202 and anode 208′ will not be relatively moreseparated progressing from the ends toward the middle of the anode 208′and cathode 202. Thus the anode 208′, as shown in FIG. 23 c is formed asa “peaked” anode. It will be understood by those skilled in the art thatthe cathode 202 could be machined as well with a “peak” in the centersuch that when the cathode 202 bows with the main insulator 204, theanode 208 will also remain relatively equidistant from the cathode 202′as shown in FIG. 23 d.

FIG. 24 shows one end of an electrode assembly 220 adapted to reduceexpanded electrode erosion at the ends of the electrodes, i.e., at theend 226 of the cathode 224, and the facing region of the anode (notshown in FIG. 24.). In the embodiment shown in FIG. 24 there is alsoshown the anode mount 230 and the ground connection 232 of the anodemount 230 to the top half of the discharge chamber 222. In additionwithin the electrode assembly 220 are positioned a plurality of currentreturn tangs 234 connected, e.g., by welding, between the chamber tophalf 222 and the anode mount 230.

Applicants have discovered that modifying the inductance in the regionof the electrode assembly near the ends 226 of the cathode 224, e.g.,lowering the inductance, e.g., by removing current return tangs 234 inthis area reduces the expanded erosion at the ends 226 of the cathode224. As shown in FIG. 24, the three tangs closest to the end 226 of thecathode 224 have been removed leaving connection stubs 240 on the tophalf of the chamber 222 and 242 on the anode mount 230.

Applicants have discovered that removing the tangs 234 at leas backalong the cathode 224 length to beyond the region of expanded electrodeend erosion reduces the end erosion effects. In FIG. 24, the connectionstub 240 furthest to the left along the longitudinal length of thecathode 224 is just beyond the extent of the expanded electrode erosionalong the longitudinal length of the cathode 224 and the first tang 234is approximately _ cm beyond this extend along the longitudinal lengthof the cathode 224.

FIG. 25 a shows a cross-sectional view of an electrode, e.g., an anodethat has been exposed to gas discharged voltages producing gasdischarges in, e.g., an ArF gas discharge laser. The anode 250 is shownin transverse cross section to comprise a main body 252, havingcontained within the main body an insert 254 in the area of thedischarge region for the anode 250. The insert 254 may be, e.g., of adifferent material than the main body 252. The anode 250 may alsocomprise a front side portion 258 in the direction from which gas flowis directed over the anode and a rear side portion 260. The insert 254may have a crown area 256 where generally the discharge is confinedduring fluorine gas discharge laser operation.

Then anode 250 illustrated in FIG. 25 a is one which has been exposed toa large number of discharges after which applicants observed that thecrown area 256, which initially was generally flat on top, hadpreferentially erodes on the left-most side of the crown 256 forminggenerally a slanted profile, slanted to the horizontal by about 4.5°.

Applicants observed that this slant is toward the side where in theparticular fluorine gas discharge laser the discharges occurred ispositioned a preionizing tube (not shown), as is well known in the art,which extends longitudinally generally parallel to the longitudinallength of the anode. Applicants believe that the differential erosion isdue to asymmetries in the gas discharge on the side of the preionizationtube due to the presence of the preionization tube and its electricaleffects on the discharge, and have also observed discharge splitting inthe discharge which appear to be related, at least in part, to sharpcorner features and/or wear features on the electrode. Applicants haveobserved a similar differential wear on the cathode, though perhaps notas pronounced. applicants also believe that asymmetries in the dischargeon one side of the discharge, particularly the left side as viewed inrelation to the anode as shown in FIG. 25, cause or at leastsignificantly contribute to the observed differential erosion.

In an effort to improve electrode life, particularly anode life, andavoid the detriments of discharge splitting, applicants propose anelectrode as illustrated schematically in FIG. 25 b. There is shown ananode 270. It will be understood that the anode 270 may be the insert,as shown with respect to element 254 in FIG. 25 a, intermediate metalfront side and rear side portions, 258,260, or may be a blade, e.g.,blade 82, e.g., as shown in FIG. 13, intermediate a front side ceramicfairing 84 and a rear side ceramic fairing 86.

The anode 270, like the insert 254 of FIG. 25 a and the blade 82 of FIG.13 has generally straight side walls 274, 276. According to anembodiment of the present invention the anode 270 may be formed having agenerally elliptical crown portion 272 which is shown to haveintersections with each of the respective sidewall portions 274,276along a radius of curvature and to be tilted, i.e., its long axis istilted to the horizontal by, e.g., about 4.5° in the direction oppositeto that of the differential erosion shown in FIG. 25 a. In this manner,the anode 270 may be formed with a beveled top that is beveled away fromthe differential erosion side providing a curvilinear upper surface thatis raise more on the side where the differential erosion will occur,e.g., the side of the asymmetry of the discharge, e.g., the side of thepreionization tube (not shown) and also avoids sharp edges at theintersection of the top portion 272 with the sidewall portions 274,276and/or due to the insert having a flat top and extending beyond theupper surfaces of the front side portion 258 and/or the rear sideportion 260. such an anode, can then serve to promote dischargestability by promoting e-field uniformities. Those skilled in the artwill understand that the cathode may be similarly constructed.

The above-described embodiments of the present invention are intendedonly for explanation and illustration purposes and are not the onlyembodiments in which the present invention may reside. Those skilled inthe art will understand that many modifications and changes may be madeto the described embodiments without changing the intent and spirit ofthe present invention. For example, while anode and cathode have beenused for the electrodes respectively that are grounded and connected tohigh voltage, in truth the cathode could be connected to a high negativeor positive voltage and thus in one case be a cathode and in another ananode, electrically speaking, with respect to the grounded electrode,herein called in this context the anode. Similarly, whatever theelectrical power configuration, typically one electrode, herein referredto as the cathode, is mounted to the housing of the fluorine gasdischarge laser, which historically is grounded, and the cathodehistorically has been the high voltage electrode, and thus must beinsulated from the chamber, e.g., the top of the chamber. Those skilledin the art will understand that it is possible to not have the chamberat ground, though not likely, and the aspects of the present inventionthat relate to modifications influenced by or dictated by theinterconnection of what is herein called the cathode and the insulatingmechanism, the main insulator, are not limited to the particularelectrode actually being electrically the cathode. Similarly thefluorine chemistry and the different interactions with fluorine and,e.g., the formation of reefs naturally and the impact of artificiallyforming reefs are independent of the denomination of the particularelectrode as a cathode or anode. They are dictated by current flow andion attractions. The use of cathode or anode in this application and inthe claims will be understood to be a convention common in the industrytoday with the grounded electrode in electrical contact with thegrounded chamber housing being referred to as the anode and the highvoltage electrode insulated from the housing being referred to as thecathode. However, equivalents of the claimed invention will beunderstood by those skilled in the art to exist where, electricallyspeaking, the anode of the present application is a cathode and viceversa. Other metals and alloys that have properties similar to thosediscussed in the present application may also form equivalentsubstitutes as those skilled in the art will appreciate.

1. A gas discharge laser having a laser gas containing fluorinecomprising: an elongated electrode body forming an arcuate or ellipticalfacing portion; a first elongated rotated V-shaped groove formed alongsubstantially all of the elongated electrode body; a second elongatedoppositely rotated V-shaped groove formed along substantially all of theelongated electrode body; the first and second rotated V-shaped groovesforming a discharge receiving ridge between the first and second rotatedV-shaped grooves.
 2. The apparatus of claim 1 further comprising: adifferentially faster eroding material filling the first and secondrotated V-shaped grooves.
 3. The apparatus of claim 2 furthercomprising: the differentially faster eroding material is a solder. 4.The apparatus of claim 1 further comprising: the differentially fastereroding material is a lead-tin solder.
 5. The apparatus of claim 2further comprising: the differentially faster eroding material is alead-tin solder.
 6. The apparatus of claim 3 further comprising: theelectrode body comprising annealed copper.
 7. The apparatus of claim 4further comprising: the electrode body comprising annealed copper. 8.The apparatus of claim 5 further comprising: the electrode bodycomprising annealed copper.
 9. A gas discharge laser having a laser gascontaining fluorine comprising: a first and a second elongated gasdischarge electrode; the first and the second elongated gas dischargeelectrodes facing each other to form a gas discharge region between thefirst and the second elongated gas discharge electrodes: at least one ofthe first and second elongated gas discharge electrodes being machinedto form a crown to receive the gas discharge that compensates for thebowing of at least one of the gas discharge electrodes during operationof the fluorine gas discharge laser.
 10. The apparatus of claim 9further comprising: one of the gas discharge electrodes is a cathodeattached to a wall of a gas discharge chamber of the fluorine gasdischarge laser and the cathode bows during operation of the fluorinegas discharge laser; and at least one of the cathode and the other ofthe gas discharge laser electrodes is machined to maintain a constantgas discharge region separation between the cathode and the otherelectrode to accommodate for the bowing of the cathode.
 11. Theapparatus of claim 10 further comprising: the cathode is machined to anapex at the center to accommodate for the bowing of the cathode.
 12. Theapparatus of claim 10 further comprising: the other electrode ismachined to an apex at generally the center to accommodate for thebowing of the cathode.
 13. A method of making a gas discharge laserhaving a laser gas containing fluorine comprising: forming an elongatedelectrode body in the shape of an arcuate or elliptical facing portion;forming a first elongated rotated V-shaped groove along substantiallyall of the elongated electrode body; forming a second elongatedoppositely rotated V-shaped groove along substantially all of theelongated electrode body; the first and second rotated V-shaped groovesforming a discharge receiving ridge between the first and second rotatedV-shaped grooves.
 14. The method of claims 13 further comprising:filling the first and second rotated V-shaped grooves with adifferentially faster eroding material.
 15. The method of claim 14further comprising: the differentially faster eroding material is asolder.
 16. The method of claim 13 further comprising: thedifferentially faster eroding material is a lead-tin solder.
 17. Themethod of claim 14 further comprising: the differentially faster erodingmaterial is a lead-tin solder.
 18. The method of claim 15 furthercomprising: the electrode body comprising annealed copper.
 19. Themethod of claim 16 further comprising: the electrode body comprisingannealed copper.
 20. The method of claim 17 further comprising: theelectrode body comprising annealed copper.
 21. A method of making a gasdischarge laser having a laser gas containing fluorine comprising:forming a first and a second elongated gas discharge electrode; thefirst and the second elongated gas discharge electrodes facing eachother to form a gas discharge region between the first and the secondelongated gas discharge electrodes: machining at least one of the firstand second elongated gas discharge electrodes to form a crown to receivethe gas discharge that compensates for the bowing of at least one of thegas discharge electrodes during operation of the fluorine gas dischargelaser.
 22. The method of claim 21 further comprising: one of the gasdischarge electrodes is a cathode attached to a wall of a gas dischargechamber of the fluorine gas discharge laser and the cathode bows duringoperation of the fluorine gas discharge laser; and at least one of thecathode and the other of the gas discharge laser electrodes is machinedto maintain a constant gas discharge region separation between thecathode and the other electrode to accommodate for the bowing of thecathode.
 23. The apparatus of claim 21 further comprising: machining thecathode to an apex at the center to accommodate for the bowing of thecathode.
 24. The apparatus of claim 21 further comprising: machining theother electrode to an apex at generally the center to accommodate forthe bowing of the cathode.